Sleep mode (or StandBy) is a state of your PC when it makes no noise, because neither processor nor hard drives are working. But in spite of it it can be woken up in a couple of seconds simply by pressing a key on keyboard or power button on PC case. To support noiseless sleep mode your computer's BIOS must be properly configured. For advanced information on this topic read about S1 and S3 Sleep Modes and ACPI
